The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to magnetic resonance angiograms which are produced using complex differences between image data sets acquired with different values of gradient field first moments.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The degree to which the net magnetic moment M.sub.z is tipped (flip angle), and hence the magnitude of the net transverse magnetic moment M.sub.t depends primarily on the length of time and the magnitude of the applied excitation field B.sub.1.
The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. In simple systems the excited spin induces an oscillating sine wave signal in a receiving coil. The frequency of this signal is the Larmor frequency, and its initial amplitude, A.sub.0 , is determined by the magnitude of the transverse magnetic moment M.sub.t and the flip angle. The amplitude, A, of the emission signal decays in an exponential fashion with time, t: EQU A=A.sub.0 e.sup.-t/T*.sub.2
The decay constant 1/T*.sub.2 depends on the homogeneity of the magnetic field and on T.sub.2, which is referred to as the "spin-spin relaxation" constant, or the "transverse relaxation" constant. The T.sub.2 constant is inversely proportional to the exponential rate at which the aligned precession of the spins would dephase after removal of the excitation signal B.sub.1 in a perfectly homogeneous field.
Another important factor which contributes to the amplitude A of the NMR signal is referred to as the spin-lattice relaxation process which is characterized by the time constant T.sub.1. It describes the recovery of the net magnetic moment M to its equilibrium value along the axis of magnetic polarization (z). The T.sub.1 time constant is longer than T.sub.2, much longer in most substances of medical interest.
The NMR measurements of particular relevance to the present invention are called "pulsed NMR measurements". Such NMR measurements are divided into a period of RF excitation and a period of signal emission. Such measurements are performed in a cyclic manner in which the NMR measurement is repeated many times to accumulate different data during each cycle or to make the same measurement at different locations in the subject. A wide variety of preparative excitation techniques are known which involve the application of one or more RF excitation pulses (B.sub.1) of varying magnitude, duration, and direction. Such excitation pulses may have a narrow frequency spectrum (selective excitation pulse), or they may have a broad frequency spectrum (nonselective excitation pulse) which produces transverse magnetization M.sub.t over a range of resonant frequencies. The prior art is replete with excitation techniques that are designed to take advantage of particular NMR phenomena and which overcome particular problems in the NMR measurement process.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G.sub.x, G.sub.y, and G.sub.z) which have the same direction as the polarizing field B.sub.0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
NMR data for constructing images can be collected using one of many available techniques. Typically, such techniques comprise a pulse sequence made up of a plurality of sequentially implemented views. Each view may include one or more NMR experiments, each of which comprises at least an RF excitation pulse and a magnetic field gradient pulse to encode spatial information into the resulting NMR signal. As is well known, the NMR signal may be a free indication decay (FID) or a spin-echo signal.
The present invention will be described in detail with reference to a variant of the well known Fourier transform (FT) imaging technique, which is frequently referred to as "spin-warp". The spin-warp technique is discussed in an article entitled "Spin Warp NMR Imaging and Applications to Human Whole-Body Imaging" by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (G.sub.y) along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient (G.sub.x) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse G.sub.y is incremented (.DELTA.G.sub.y) in the sequence of views that are acquired during the scan to produce a set of NMR data from which an entire image can be reconstructed.
There are a number of well known NMR techniques for measuring the motion, or flow of spins within the region of interest. These include the "time-of-flight" method in which a bolus of spins is excited as it flows past a specific upstream location and the state of the resulting transverse magnetization is examined at a downstream location to determine the velocity of the bolus. This method has been used for many years to measure flow in pipes, and in more recent years it has been used to measure blood flow in human limbs. Examples of this method are disclosed in U.S. Pat. Nos. 3,559,044; 3,191,119; 3,419,793 and 4,777,957.
A second flow measurement technique is the inflow/outflow method in which the spins in a single, localized volume or slice are excited and the change in the resulting transverse magnetization is examined a short time later to measure the effects of excited spins that have flowed out of the volume or slice, and the effects of differently excited spins that have flowed into the volume or slice. Examples of this method are described in U.S. Pat. Nos. 4,574,239; 4,532,474 and 4,516,582.
A third technique for measuring motion or flow relies upon the fact that an NMR signal produced by spins flowing through a magnetic field gradient experiences a phase shift which is proportional to velocity. This is referred to in the art as the "phase modulation" technique. For flow that has a roughly constant velocity during the measurement cycle the change in phase of the NMR signal is given as follows: EQU .DELTA..phi.=.gamma.M.sub.1 v
where M.sub.1 is the first moment of the magnetic field gradient, .gamma. is the gyromagnetic ratio and v is the velocity of the spins. To eliminate errors in this measurement due to phase shifts caused by other sources, it is common practice to perform the measurement at least twice with different magnetic field gradient moments as described in U.S. Pat. No. 4,609,872. The difference in phase at any location between the two measurements is then as follows: EQU .DELTA..phi.=.gamma..DELTA.M.sub.1 v
By performinq two complete scans with different magnetic field gradient first moments and subtracting the measured phases in the reconstructed image at each location in the acquired data arrays, a phase map is produced which accurately measures the velocity of constantly moving spins.
Magnetic resonance angiograms are produced by acquiring and calculating the phase difference between at least two NMR data sets, each with a different value for the first moment of a magnetic field gradient. The phase is substantially the same in the two data sets at locations where the spins are stationary and such tissues appear dark in the reconstructed image. On the other hand, moving spins impart a phase to the acquired NMR signal which is proportional to velocity and the value of the first moment of the magnetic field gradient. Since the first moments are different in the two acquired NMR data sets, moving spins will produce a phase difference and the locations where these phase differences occur will appear bright in the angiogram image.
There are a number of methods currently used to produce phase difference angiogram data from two or more acquired NMR data sets. In one method, which is referred to as the "phase difference method" the difference in phase between corresponding elements in the acquired NMR images is calculated and this phase difference is used to control the intensity of the corresponding image pixel. In a second method, which is referred to as the "complex difference method", the in-phase and quadrature components of corresponding elements in two acquired NMR raw data sets are subtracted to produce complex difference data which is then Fourier transformed into the image domain. Alternatively and equivalently, the two raw NMR data sets can be Fourier transformed, and the in-phase and quadrature components in corresponding image elements can be subtracted. In any case, the complex difference image is used to reconstruct the angiogram image. There are subtle differences between the angiograms produced by the phase difference and complex difference methods which make them both uniquely useful in clinical applications.
The accurate reconstruction of a magnetic resonance angiogram using either of these processing methods assumes that there are no phase errors in the measurements. While there are many sources of phase errors, most phase errors are the same in the two acquired data sets and are subtracted out of the image data by the difference process. However, some phase errors may be different in the two acquired data sets and the difference process used to produce an angiogram will produce artifacts in the reconstructed image.
There are a number of methods known for correcting phase errors in NMR measurements. For example, Juwhan Liu et al describe a phase correction technique in the Journal of Magnetic Resonance, Vol. 86 pp 593-604 (1990) in an article entitled "An Automatic Phase Correction Method In Nuclear Magnetic Resonance Imaging" which reduce errors due to such system hardware problems as (a) misadjustment of the reference phase relative to the receiver phase detector; (b) phase shifts caused by noise filters; (c) incorrect alignment of data acquisition window; (d) imperfect selective RF excitation pulses; (e) amplifier dead time; and (f) eddy currents. Other phase correction methods are described by C. B. Ahn et al in an article in IEEE Transactions On Medical Imaging, Vol M1-6 No. 1March 1987 entitled "A New Phase Correction Method In NMR Imaging Based On Autocorrelation And Histogram Analysis"; and by M. A. Bernstein et al in an article in Medical Physics, Vol. 16 No. 5, Sept/Oct 1989 entitled "Improved Detectability In Low Signal-To-Noise Ratio Magnetic resonance Images By Means Of A Phase-Corrected Real Reconstruction". None of these techniques have been employed to correct angiograms produced by the complex difference method.